A type of electrical resistance material which has recently come into commercial use is a vitreous enamel resistance material which comprises a mixture of a glass frit and finely divided particles of an electrical conductive material. The vitreous enamel resistance material is coated on the surface of a substrate of an electrical insulating material, usually a ceramic, and fired to melt the glass frit. When cooled, there is provided a film of glass having the conductive particles dispersed therein. Terminations are connected to the film to permit the resultant resistor to be connected in the desired circuit.
The materials which have been generally used for the conductive particles are the noble metals. Although the noble metals provide vitreous enamel resistance materials which have satisfactory electrical characteristics, they have the disadvantage that they are expensive. Thus, the resistors made from the vitreous enamel resistance materials containing the noble metals are expensive to manufacture. Therefore, it would be desirable to have a vitreous enamel electrical resistance material which utilizes a relatively inexpensive conductive material so as to provide an electrical resistor which is relatively inexpensive to manufacture. In addition, the conductive material used must be capable of providing a resistance material having a wide range of resistance values and which is relatively stable over the entire range of the resistance values. By being stable it is meant that the resistance value of the resistance material does not change or changes only a small amount under operating conditions, particularly when subjected to changes in temperature. The change in resistance value of an electrical resistor per degree change in temperature is referred to as the "temperature coefficient of resistance" of the resistor. The closer the temperature coefficient of resistance is to zero, the more stable is the resistor with respect to changes in temperature.